Hydrolysis by myosin A of several synthetic adenosine triphosphate analogues

The behaviours of three synthetic ATP analogues, 6-dimethylamino-9-β- d -ribofuranosylpurine 5′-triphosphate (dimethyl-ATP), 9-(4′-hydroxybutyl)-6-aminopurine 4′-triphosphate (BTP) and ribose 5-triphosphate (RTP), on hydrolysis by myosin A were investigated mainly in 0.5 M KCl and 5 m M CaCl 2 (pH7.0, 20°), together with those of ATP, ITP and inorganic tripolyphosphate (TP i ). The following results were obtained. 1. 1. The effect of divalent cations, such as Mg 2+ , Mn 2+ , Ca 2+ , Sr 2+ and Ba 2+ , on the rate of hydrolysis of BTP, TP i subtrates, the relations between the ionic radii ( r ) of various added divalent cations and the rates of hydrolysis were expressed by a bell-shaped curve, which showed a maximum at r = 0.95 A . 2. 2. The Michaelis constants of hydrolysis of ITP, TP i , BTP, RTP, dimethyl-ATP and ATP were 3, 2.5, 2.5, 1.7, 1.4 and 0.2 and m M , respectively, while the ratios of the maximum velocity of hydrolysis of ITP, BTP, dimethyl-ATP, TP i and RTP to that of ATP were, 7, 3, 1.3, 1 13.5 and 1 20 , respectively. 3. 3. The Michaelis constant and the maximum velocity of hydrolysis of TP i showed a remarkable increase with decreasing pH, while the rate of hydrolysis of RTP depended slightly on the pH. The dependence of the rate of hydrolysis of BTP on the pH showed a slight depression at the neutral pH and its Michaelis constant was independent of the pH. 4. 4. p -Chloromercuribenzoate and EDTA inhibited the hydrolysis of TP i , RTP, ITP, dimethyl-ATP and BTP, although they activated ATPase. These results have been interpreted on the basis of the reaction between myosin A and nucleoside triphosphates which has been proposed by one of the present authors 7,24 and is further discussed below.